This invention relates to a process for the conversion of hydrogen-containing silane impurities to non-hydrogen containing silanes to facilitate the isolation and removal of these impurities from the desired organosilane product.
High-purity difunctional organosilanes, particularly diorganodihalosilanes, are needed for the preparation of organopolysiloxanes utilized in the manufacture of silicones. For example, the preparation of high-quality, high-performance silicone elastomers require that the difunctional organosilane contain a minimum level of trifunctional and tetrafunctional materials, to levels of a few parts per million on a molar basis.
Hydrogen-containing silane materials are a potential source of additional functionality. The hydrogen atom on a silane molecule is susceptible to cleavage, especially under the basic conditions employed during the production of siloxane materials. For the purposes of the instant invention, the term "functionality" is used to describe the ability of silane species to form a linear structure (difunctionality) a branched structure (trifunctionality) or a network structure (tetrafunctionality).
In the preparation of high-performance silicone elastomers, difunctional silane monomers with a minimum of trifunctional and tetrafunctional materials is a necessity. For example, (CH.sub.3).sub.2 SiCl.sub.2 with (C.sub.2 H.sub.5)HSiCl.sub.2 impurity would result in a dimethyl siloxane polymer with hydrogen-containing siloxane units. Cleavage of the H atom would result in a branch (trifunctionality) on the siloxane chain. Such a branch can reduce the desired physical characteristics of the silicone elastomer subsequently formed from the polymer.
Organosilanes are typically manufactured by a direct process of reacting an organic halide with silicon in the presence of a catalyst. The resultant mixture is conventionally separated into the individual species by distillation. In many instances, the boiling points of individual organosilane components are very close, creating a very difficult distillation operation. An example of such a combination is the mixture of dimethyldichlorosilane, (CH.sub.3).sub.2 SiCl.sub.2, and ethyldichlorosilane, (C.sub.2 H.sub.5)HSiCl.sub.2. The boiling points of dimethyldichlorosilane and ethyldichlorosilane are approximately 4.degree. C. apart. Reducing the ethyldichlorosilane content of dimethyldichlorosilane would require a distillation column of more than one hundred theoretical distillation trays and reflux ratios in the range of 100:1.
Sommer et al., J. Org. Chem. 32:2470-2472 (1967), discloses that organosilicon hydrides react with hydrogen halides in the presence of Group VIII metals to form organosilicon halides and hydrogen. Sommer studied the reaction of (C.sub.2 H.sub.5).sub.3 SiH with HCl to produce (C.sub.2 H.sub.5).sub.3 SiCl. Sommer does not disclose or suggest the use of this reaction in a process for the purification of organohalosilanes.
Tolentino, U.S. Pat. No. 4,421,926, issued Dec. 20, 1983, discloses the co-alkoxylation of halosilanes to facilitate separation of close-boiling halosilane materials difficult to separate by conventional distillation. Tolentino discloses that all halosilanes will be converted to alkoxysilanes. Nowhere does Tolentino demonstrate or suggest the reaction of hydrogen-containing silanes. Tolentino neither discloses or suggests the preferential reaction of hydrogen-containing silanes, with no concurrent reaction of the non-hydrogen containing halosilane, to facilitate isolation and recovery of the desired organohalosilane.